1. Technical Field
This invention relates to methods of stimulating and recording from the nervous and muscular system of the human body through the use of implantable devices based on semiconductor diodes.
2. Description of Related Art
There is interest in methods of directly stimulating bioelectrically excitable tissues by artificial means since this allows their function to be evoked or modified, thus providing a therapeutic or otherwise desirable biological effect. For example, neurostimulation may be used for restoring function in cases of neural injury or disease. Neurostimulation in this context refers to the stimulation of electrically excitable tissues of living things. This can include, for example, the human tissues of the brain, heart, muscle, and nervous system.
There is also interest in recording tissue bioelectrical events. Tissue bioelectrical events arise from the flow of ionic currents as a result of the action of cellular ionic pumps and channels, which underlie the bioelectrical activity of neural and muscle tissues in the body. These neural and muscle tissues are associated with the function of the brain, muscles, and nervous system. The ionic currents are well known and are used for electro-cardiograms, electroneurograms, and electromyograms.
A common method of neurostimulation is the application of pulsed electrical currents directly to tissue through electrodes implanted within tissue or indirectly through the body surface.
Electrical currents applied to tissue are known to affect the membranes of excitable cells, causing a depolarizing effect that can lead to a cell action event that depends on its type and biological function. The pulsing of currents is needed to prevent accommodation to current flows and to fulfill certain physiologic conditions that enables electricity to be effective.
It is also possible to apply electrical currents to the body surface in which case they diffuse in the volume conductivity of tissue and attenuate according to well known laws. These currents can also stimulate near-surface nerves and muscle tissues to some degree. but cannot reach deeper tissues because of high electrical losses in tissue and the rise in the needed current levels to above those that would cause electrical shock.
The strong diffusion of electrical current in tissues from surface electrodes means that specific stimulation of a given nerve or nerve fiber within a bundle is very difficult and rather there is a tendency for electrical currents applied to the body surface to broadly stimulate in undesirable ways. Implantable electrodes overcome these problems but are invasive and suffer from the undesirable need to either run wires through the skin or work with relatively bulky implanted power systems that run on batteries or are powered by external radiofrequency (RF) powering techniques.
Technologies that deliver electrical currents to tissues by way of RF induction to an implanted device are well known to the art. In general these approaches use an inductor implanted within the body to magnetically couple to an external RF field. Often times this inductor is coupled with a capacitor to form a resonant circuit that is more efficient in coupling to applied RF energy. These devices are relatively large and can be on the order of a centimeter in size. A discussion of methods of coupling energy to implanted RF devices was published by Heetderks (1988) and an overview of the current state of the art and sizes of neuroimplants by Wise et al. (2004) and by others is incorporated herein by reference. The Heetderks paper mostly confines itself to power induction at frequencies below about 50 MHz. The inventors find however that higher frequencies in the hundreds of megahertz and into the microwave region above about 500 MHz are also being used in some designs.
High frequency currents are not known to stimulate bioelectrically excitable tissues of the nervous system of the body because they are faster than physiologic events can respond. As long as they are relatively high frequency, above several tens of kilohertz and continuing up into the megahertz region currents do not stimulate bioelectrical events or sensations of pain.
A major concern in the development of neurostimulators for implantation near nerve or muscle for therapeutic applications in the human body is the size of the implant. It is preferable that the implanted devices be small and perhaps something that could be introduced into the body through minimally invasive methods, such as syringe needle injection. This is not only for ease of insertion into tissues, but so that they produce less complications such as pressure or force against sensitive tissues as a person moves or exercises. There is also less immunological response and inflammation of tissues with small devices as it reduces their attendant risk of complications. This feature tends to encourage more widespread use in situations which are elective rather than critical.
A neurostimulation device known as a Bion™ has been described by Loeb et al. which is an example of present methods of designing implantable neurostimulation devices. It is a small cylindrical electrical device which derives its energy from an externally applied RF field. As presently designed, the size of these devices ranges from 6 mm to about 1.5 cm. These devices incorporate active LSI logic and inductive RF powering.
Some versions store energy in batteries or capacitors to deliver later upon digital command and so provide electrical pulses through integral electrodes to neural tissues. These devices are targeted for therapeutic stimulation of muscle and nerves by being implanted within body tissues and in some cases are used for pain relief, treating urinary incontinence, and can be programmed to actuate nerves and muscles in the restoration of lost function in limbs. A disadvantage of these devices is their relative complexity and large size. The large size limits their medical applicability to situations where they can be introduced by surgery or through a large trocar.
This application incorporates by reference provisional patent application Nos. 60/916,152 filed on May 4, 2007 and 61/093,546 filed on Sep. 2, 2008 in their entirety.